McIlroy had two eagles on the back nine to go with his seven birdies as he carded a nine-under-par 63 at Muirfield Village Golf Course in Ohio on Thursday.
The Northern Irishman, fresh from his BMW PGA Championship win in England on Sunday, is three shots clear of Englishman Paul Casey and American pair Chris Kirk and Bubba Watson despite double-bogeying the 14th hole.
Though, it was not all smiles for McIlroy, who appeared to sustain a knee injury at the seventh hole.
“I think my foot might have just got stuck in the ground a little bit and tweaked my knee,” McIlroy told the Golf Channel.
“I’m going to go and get some treatment done now and have a physio look at it.
“It’s painful right after I hit shots and then I walked it off and it would be OK, but then it would be painful again (after the next shot).
“With a little bit of ice and some treatment, hopefully it will be alright for tomorrow.”
A shot further back were Americans Keegan Bradley, Michael Thompson and JB Holmes, who all carded 67s to be five-under overall.
As for world number one and Crowne Plaza Invitational champion Adam Scott, he posted a three-under 69 to be six shots off the pace heading into the second day.
McIlroy hit the headlines after he called off his engagement to tennis star Caroline Wozniacki last week, though he responded in tremendous fashion by taking out the European Tour’s showpiece event in Surrey.
The two-time major winner carried that good form into the Memorial, going bogey-free on the front nine, on which he produced four birdies.
McIlroy holed a birdie at the 10th before following that up with the first of his two eagles, a 27-foot putt at the par-five 11th.
The 25-year-old added two more birdies and an eagle, but shot a six on the par-four 14th to blot his card.
American pair Hunter Mahan, Ryan Moore and Justin Leonard all had rounds of 68 to be tied for eighth spot at four under.
Meanwhile, American veteran Phil Mickelson shot a bogey and two double bogeys from his final three holes to be nine shots off the lead.
Mickelson had been in the hunt for most of the day after going blemish-free on the front nine but he faltered badly late on to card an even-par 72.
For more sports news visit allSports.com.gh
This article has 0 comment, leave your comment.